Soak the clams in water for about six hours, changing the water every two hours or so, to expel the sand inside the shells. Drain and rinse several times until the liquid is clear and free from traces of sand. Drain well.
If your mushrooms are large, tear or cut into bite-size pieces.
Heat six cups of water in a pot.
Place the miso paste in a heat-proof bowl. Stir to loosen.
When the water boils, take scoop cup and pour into the miso paste. Whisk until smooth.
Dump the drained clams and mushrooms into the boiling water.
Once the clams open, turn off the heat.
Stir in the miso paste and half of the scallions.
Taste and add salt, if needed.
Ladle the clams miso soup in bowls and top with the remaining scallions.